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P0400: Exhaust gas recirculation (EGR) flow malfunction

The ECU sees that EGR valve flow is not what it expects. It is usually carbon build up in the valve or passages, or a control problem.

Code P0400 at a glance
CodeP0400
SystemEngine and transmission
GroupP04xx, Emissions control
TypeGeneric: it means the same on every make
Can you keep driving?With care

What P0400 means

The EGR valve feeds some exhaust gas back into the intake to lower combustion temperature and emissions. The ECU compares what it asks for with what it measures, and the figures do not match. The code does not say whether flow is too low or too high, only that something is wrong.

Most common causes

  • EGR valve dirty or stuck with carbon
  • Blocked EGR passages or cooler
  • Cracked or loose vacuum hose to the EGR (vacuum operated valves)
  • Damaged connector or wiring on the valve
  • Faulty EGR valve or its position sensor
  • Faulty vacuum solenoid controlling the EGR

Symptoms

  • Engine warning light on
  • Hesitation or rough idle
  • Loss of power
  • More smoke on diesels

Can you drive with P0400?

With care You can drive, but the engine may run worse and go into limp mode. Have it checked soon so it does not get worse.

How to check it

With your reader, watch the commanded and actual EGR position live if available: if they differ, the valve is not responding. On engines with an airflow meter, compare air flow with the EGR open and closed.

How to read and clear P0400 with Final OBD

Plug an OBD2 scanner in under the steering wheel, switch the ignition on and open Final OBD. Under Faults you’ll see P0400 with this same explanation and whether it’s confirmed, pending or permanent.

With Free you read this code with its short description for nothing, and clear it if the engine stores it over standard OBD. With Pro you get the full explanation, how the car was running when it was logged (revs, temperature, load) and clearing in every control unit.

Clearing the code only turns the light off: if the cause is still there, it comes back. Clear it once it’s fixed, and not right before the ITV. It’s all in how to read and clear fault codes and in the ITV and OBD.

Similar codes

P0400 is a generic code, so it means the same on any make, for example on Abarth, Citroën, Cupra, Hyundai, Mini, Seat. See what Final OBD does on your car in makes and models.

What does code P0400 mean?

The EGR valve feeds some exhaust gas back into the intake to lower combustion temperature and emissions. The ECU compares what it asks for with what it measures, and the figures do not match. The code does not say whether flow is too low or too high, only that something is wrong.

Can you drive with P0400?

You can drive, but the engine may run worse and go into limp mode. Have it checked soon so it does not get worse.

How do you clear P0400?

With an OBD2 scanner and an app like Final OBD, which clears it for free if it’s stored by the engine control unit. But clearing it only turns the light off: if the cause isn’t fixed, P0400 comes back.